With the emergence of multiple radio frequency bands and multiple modulation modes available for wireless communications, there is a need for transmitting radio frequency (RF) communication signals within multiple frequency bands and multiple modulation modes. For example, the emergence of various communications standards has driven the need for wireless communication devices, such as wireless telephones, wireless personal data assistants (PDAs), pagers, two-way radios, and other types of wireless devices, to be compatible with multiple standards employing multiple modulation modes and multiple radio frequency bands. Having multimode and multiband capability allows a wireless device to operate with more than one system or standard, and, depending on the system available, the user can potentially use the device on more than one wireless communication system.
A multimode communication device is designed to transmit and receive RF communication signals of different modulation schemes depending on the communication systems' modulation schemes in operation in the user's geographical location. For example, modulation schemes for digital RF signals may include time division multiple access (TDMA) schemes, code division multiple access (CDMA) schemes, global system for mobile communications (GSM) based schemes, second generation wireless data technology schemes (2G, including intermediate standards such as 2.5G), third generation (3G) wireless data technology schemes, multiple access schemes such as Enhanced Data Rates for Global Systems for Mobile Communications Evolution (EDGE), or other suitable technology schemes.
A multiband and multimode wireless device incorporates electronics necessary to operate within a plurality of systems using multiple frequency bands and multiple modulation nodes. For example, the transmitter stage of a multiband and multimode wireless device can be similar to the transmitter stage of a single band wireless device designed to transmit within one frequency band, but is adapted to transmit at multiple frequency bands. Accordingly, a dual band and dual mode wireless device has the capability to operate in two different modes at two different frequency bands, such as one band for GSM, and one band for EDGE. This allows a user to move from one system to another, as long as the multiband and multimode wireless device supports the standard for the system in operation.
As shown in FIG. 1, a typical multiband transmitter stage 10 may include a synthesizer 12, a single shared voltage controlled oscillator (VCO) 24, a multiband transmitter circuit 14, a selector switch 16, a power amplifier (PA) 18, and a dual band antenna 20 to transmit a transmitter output signal 22, that, at different times, is at different frequencies in different bands. Single shared VCO multiband transmitter circuit 14 includes a single shared VCO 24, a VCO buffer 26, a first divide-by-two circuit 28, and a second divide-by-two circuit 32. The single shared VCO 24 produces a VCO output frequency signal 25. VCO buffer amplifier 26 buffers the VCO output frequency signal 25. First divide-by-two circuit 28 produces a high band output signal/synthesizer feedback signal 30 and the second divide-by-two circuit 32 produces a low band output signal 34. Selector switch 16 switches between the low band output signal 34 and the high band output signal/synthesizer feedback signal 30 to produce an output signal 36 for amplification by the power amplifier (PA) 18 and subsequent transmission by the dual band antenna 20.
Alternatively, the low band output signal 34 and the high band output signal/synthesizer feedback signal 30 may be coupled to separate PAs for amplification and then combined for transmission over dual band antenna 20 or over separate high and low band antennas. The single shared VCO multiband transmitter circuit 14 employs the single shared VCO 24 and therefore employs a single “transmitter path” because the output of the single shared VCO 24 covers transmission in the multiple frequency bands.
Synthesizer 12 includes a phase detector 40, a loop filter 42 and a frequency divider 44. Synthesizer 12 produces a transmitter input signal 46 as a tuning voltage to the single shared VCO 24 of multiband transmitter circuit 14. Phase detector 40 receives reference signal 48 and a divided synthesizer feedback signal 50 for producing a phase detector output signal 52. Loop filter 42 receives the phase detector output signal 52 to produce the transmitter input signal 46 to provide the tuning voltage to single shared VCO 24, as previously stated. The single shared VCO multiband transmitter circuit 14 provides the high band output signal/synthesizer feedback signal 30 to synthesizer 12 as a feedback signal to form a phase locked loop. Although multiband transmitter circuit 14 is described herein as including the single shared VCO 24, the VCO buffer 26, the first divide-by-two circuit 28, and the second divide-by-two circuit 32, the single shared VCO multiband transmitter circuit 14 may be considered part of synthesizer 12 to form the phase locked loop circuit, as known in the art. Synthesizer 12 may be used to apply GMSK (Gaussian Mean Shift Keying) modulation as known in the art.
The single shared VCO 24 may, depending on the circuit, reduce the amount of circuitry required when compared to a multi-VCO circuit for a multiband transmitter, resulting in a more compact transmitter within, for example, a wireless communication device. This approach may require minimal design effort, since a single VCO and VCO transmitter path is employed. As a result, the cost and complexity of the multiband transmitter stage 10 may be reduced by using a single shared VCO 24 for the multiband transmitter circuit 14. The single shared VCO 24 for the single shared VCO multiband transmitter circuit 14 is also used because many complementary wireless telecommunications standards in the 800/900 MHz band and in the 1800/1900 MHz band, for example, conveniently allow for the use of a single shared VCO 24 by using the first and second divide-by-two circuits 28, 32 since the 1800/1900 MHz band is a multiple of two for the 800/900 MHz band.
However, using a single shared VCO 24 compromises power consumption, noise performance and other performance criteria for the multiband transmitter circuit 14. Firstly, the single shared VCO multiband transmitter circuit 14 is required to meet the necessary noise criteria when functioning in each of the multiple frequency bands. As a result, the power consumption of the single shared VCO multiband transmitter circuit 14 when operating with the more stringent noise criteria for one band, such as in the 800 MHz or 900 MHz band, can result in unnecessarily high power consumption of the single shared VCO multiband transmitter circuit 14 when operating in a frequency band such as in the 1800 MHz or 1900 MHz band, where the noise criteria may be relatively less stringent. Therefore, either power consumption is increased so that the single shared VCO multiband transmitter circuit 14 meets the required noise criteria for the band with the most stringent noise criteria, or the noise criteria in the band having the more stringent noise criteria is not met if power consumption is lowered to meet the noise criteria of the band having less stringent noise criteria. Consequently, the single shared VCO 24 for the single shared VCO multiband transmitter circuit 14 configuration may not operate at optimal power consumption and noise performance levels.
A second method for achieving multiband transmission capability is to utilize a single shared VCO 24 that is switched via a switching circuit and two band-pass filters operating in different bands between two transmitter paths. Although this approach uses only a single shared VCO 24, adding the switching circuit and band-pass filters can add insertion loss, thus decreasing a level of the transmitter output signal 22 and increasing the generation of transmitter circuit noise. In addition, the cost and complexity of the shared single VCO 24 for the single shared VCO multiband transmitter circuit 14 can be exacerbated when configured with a switching circuit and band-pass filters.